1. Field of the Invention
The present invention relates to a ferritic stainless steel having remarkably high toughness both in the base metal and the weld zone. Particularly, the steel of the present invention possesses excellent low temperature toughness and high workability.
2. Description of the Prior Art
As is well known, a ferritic stainless steel has lower elongation and impact strength as compared with an austenitic stainless steel, and this tendency increases as the chromium content increases. For example, when they are compared in respect of the transition temperature (vTrs) of impact value, a commonly used 12 Cr ferritic stainless steel (AISI 405) shows -10.degree. C, while another commonly used 17 Cr ferritic stainless steel (AISI 430) shows about 20.degree. C. Also, when the chromium content is higher, the 475.degree. C embrittlement, the .sigma. embrittlement and the high temperature embrittlement are caused. These embrittlements are practically negligible if the chromium content is at a lower level as seen in the 405 and 430 grades, but their effects are remarkably worse in the case of higher Cr grades, such as, a 25 Cr stainless steel (AISI 446).
The high temperature embrittlement is caused by excessive grain growth as the result of dissolution of Cr carbides (Cr.sub.23 C.sub.6) at high temperatures (1150.degree. C or higher) into the steel matrix to form a single high chromium ferrite phase. This phenomenon is often seen even in the case of 430 grade steels in the welding heat-affected zone and deposited metal zone which were heated to 1150.degree. C or higher. In fact, the Cr.sub.23 C.sub.6 that precipitates in the grain boundaries in the case of a 430 steel is in the form of elongated strips which cause decreasing low temperature toughness and weld cracking.
The toughness of the weld zone can be mostly recovered by annealing between 730 and 790.degree. C, but in the case of a steel in which the grains have grown excessively coarse, the recovery is small. For this reason, in common practice, the heat input is maintained as low as possible so as to restrict the excessive grain growth.
Ordinarily, in order to maintain the toughness of the weld zone of a ferritic stainless steel, it is necessary to prevent the growth of ferrite grains and at the same time to use a steel material with a chromium content maintained as low as possible for suppressing the high temperature embrittlement. However, use of the steel material with a lowered chromium content induces martensitization of the steel due to the formation of austenite, and depending on the carbon content, it inevitably brings forth hardening of the steel due to martensite. Therefore, various means have been tried, such as, the addition of Ti, Nb, etc., to overcome this problem. But in the case of a stainless steel with a higher chromium content than that of AISI 430, the steel has a greater tendency to form a single ferrite phase when Ti is added because Ti fixes the carbon, although the addition is effective for stabilizing the carbon, so that it is impossible to prevent the grain growth and thus the steel is susceptible to embrittlement.
As shown in Table 1, the impact value of AISI 430 steel is about 6 kg-m/cm.sup.2 at room temperature (25.degree. C) while the impact value in a weld zone of the same steel is very low, such as 0.6 kg-m/cm.sup.2. Whereas, the 430 steel with the addition of Ti for the above purpose possesses an impact value of 16 kg-m/cm.sup.2 at room temperature and its weld zone shows an impact value of 17.8 kg-m/cm.sup.2. Thus, a remarkable improvement of toughness both in the base metal and the weld zone is obtained but almost no improvement with respect to the transition temperature is realized, namely +7.degree. C in the weld zone and +15.degree. C in the weld zone. Particularly, at a low temperature, e.g., lower than -20.degree. C, this steel possesses an impact value of 1 kg-m/cm.sup.2 in the base metal and 0.5 kg-m/cm.sup.2 in the weld zone and thus almost no improvement of toughness is obtained.
TABLE 1 ______________________________________ Results of V-Charpy Test of Ferritic Stainless Steels ______________________________________ Impact Value of the Base Metal (kg/cm.sup.2) Temperature (.degree. C) -25 0 25 50 75 ______________________________________ Sample No. 1 2.8 6.3 8.8 11.2 11.9 2 1.0 2.0 6.0 9.0 10.0 3 0.9 2.3 16.0 17.5 17.5 Remarks Corres- Impact Value of the Weld Zone (kg/cm.sup.2) Ponding Temperature (.degree. C) -25 0 25 50 75 AISI grade ______________________________________ 1 2.0 1.6 2.9 7.2 6.8 405 by literature 2 0.5 0.5 0.6 0.7 0.9 430 3 0.5 0.4 17.8 18.5 18.7 Ti added 430 ______________________________________
Measurements are obtained from sub-sized (t=5) test pieces.
As described above, it is understood that the addition of Ti to a ferritic stainless steel has a remarkable effect on the toughness at room temperature, but it does not contribute to the improvement of the toughness at low temperature.